A heat-sensitive wire is obtained such that a thermistor made of a polymer compound is buried between coaxial internal and external conductors, and they are drawn to have a linear shape. In this heat-sensitive wire, an impedance between the conductors is varied according to a temperature. The impedance is low at a high temperature, and the impedance is high at a low temperature. A temperature detector is formed using the above characteristics of the heat-sensitive wire.
The impedance of the heat-sensitive wire is mainly constituted by a resistance component obtained by moving ions contained in a polymer compound and an electrostatic capacitance between the conductors. When the heat-sensitive wire is used as a temperature measuring element, an error occurs in a measurement value due to localization of ions or charge accuraulation in a capacitor, and the degradation of the heat-sensitive wire is accelerated. For this reasons, an ac voltage having symmetrical positive and negative components must be applied to the heat-sensitive wire, and a commercial power source having a sine wave is conventionally used to measure the impedance. For example, the following method is described in Japanese Patent Laid-Open No. 59-44803. That is, an ac voltage is applied to a temperature detecting wire, and temperature detection is performed by the magnitude of the impedance of the temperature detecting wire to control a heating power.
An impedance z of the heat-sensitive wire can be represented by a vector. When the above arrangement is regarded as a series equivalent circuit, assuming that the angular velocity of a power source frequency is represented by .omega.; a resistance component, R; and an electrostatic capacitance, C, the impedance Z can be represented by the following equation: ##EQU1## In this equation, R and C of the right-hand side are changed in accordance with a temperature. As a result, Z of the left-hand side has a value corresponding to the temperature.
Although the heat-sensitive wire has also an inductance component as a matter of course, when the heat-sensitive wire has a practical length (several meters to several tens of meters) , the magnitude of the inductance component is smaller than that of the resistor component or the electrostatic capacitance enough to be negligible. Therefore, the inductance component is omitted in equation (1).
An absolute value .vertline.Z.vertline. of the impedance Z is calculated by equation (1) as follows: ##EQU2## In a conventional temperature detector of this type, a temperature is detected by the following means. That is, the impedance of a heat-sensitive wire is calculated as the absolute value .vertline.Z.vertline., and an absolute value .vertline.I.vertline. of a current I obtained by applying a voltage E to the heat-sensitive wire is calculated. A voltage obtained by causing the current I to flow into the heat-sensitive wire directly or through a resistor having a known resistance is converted into a dc voltage, and the current or voltage is compared with a dc reference current or voltage obtained independently of the measured voltage or current.
The impedance of the heat-sensitive wire will be described below in detail.
The section of the heat-sensitive wire is shown in FIG. 1. Assume that the length of a heat-sensitive wire 1 is represented by L; the outer radius of an internal conductor 101, a; the inner radius of an external conductor 102, b,; the specific resistance of a polymer compound 103, .rho.; and a permitivity, .epsilon., and assume that a terminal effect is neglected. In this case, a resistance component R and an electrostatic capacitance C are obtained by the following equations: ##EQU3##
On the other hand, when the heat-sensitive wire is applied to an electric blanket or the like, the flexibility of it is an important property. The internal conductor 101 is obtained by winding a thin wire or a conductive ribbon around a core made of synthetic fibers, and the external conductor 102 has the same structure as that of the internal conductor 101. Therefore, the outer radius a and the inner radius b of the conductors cannot be easily processed with high dimensional precision in the manufacturing process, and the impedance Z and its absolute value .vertline.Z.vertline. represented by equations (1) and (2) obtained as results of the resistance component R and the electrostatic capacitance C represented by equations (3) and (4) are considerably varied. For example, when the length L is 5 m, a precision of .+-.30% is obtained, and even when the length is 10 m, only a precision of .+-.20% is obtained. For this reason, in practical use, after the heat sensitive wire is cut to have a predetermined length, the cut wire is discriminated, or the cut wire is connected to a detector to determine whether the cut wire is proper in practice.
In Japanese Patent Laid-Open No. 60-125533, the following method is described. That is, a temperature is measured by a relationship between the magnitude of a current supplied to a power cable and the phase difference between a voltage applied to the power cable and the current. When this measuring method is applied to a heat-sensitive wire, since the phase difference between the voltage and the current is defined by the specific resistance permitivity of a thermistor material independently of the size of the heat-sensitive wire, temperature detection can be accurately performed regardless of the dimensional precision of the heat-sensitive wire. A measuring method in which the phase difference between the voltage and the current is detected by detecting an interval between zero-cross points of each waveform is disclosed, in e.g., Japanese Patent Laid-Open No. 51-3275.
However, extensive studies of the present inventor found that it was very important to apply an ac voltage having symmetrical positive and negative components to the heat-sensitive wire, and that even when a slight unbalanced component was present, a measurement error occurred, or the thermistor material of the heat-sensitive wire was degraded to increase the measurement error. In relation to this point, it was found that in the well-known method (for example, Japanese Patent Laid-Open No. 54-136877) in which the phases of an ac waveform were detected by detecting zero-cross points of the waveform, the occurrence of the unbalanced component could not be completely eliminated.